1. Field of the Invention
The present invention relates to a sublimation furnace and methods for preparing bulk silicon carbide with low defect density.
2. Description of the Related Art
Silicon carbide (SiC) has gained significant interest in recent years due to its outstanding chemical, physical, and electrical properties. In particular, bulk single crystal SiC has been found to be useful in semiconductor applications, including, for example, as a substrate for material for components in power electronics, and LEDs. Other applications for this material are also emerging.
Silicon carbide can be prepared by a variety of methods known in the art. For example, large single crystals of silicon carbide have been prepared using a physical vapor transport (PVT) method. For this method, a source, such as powdered silicon carbide, is provided in a high temperature region of a crystal growth furnace and heated. Also, a seed, such as a silicon carbide single crystal wafer, is provided in a lower temperature region. The silicon carbide is heated to sublime, and the resulting vapors reach the cooler silicon carbide seed upon which material is deposited. Alternatively, the source can be a mixture of silicon and carbon particles, which, upon heating, react to form SiC that subsequently sublimes and recrystallizes on the seed.
While large boules of silicon carbide can be produced using a crystal growth furnace, the process is often difficult to control. For example, it is critical that the process conditions, such as the temperature gradient between the source and the seed, be held constant throughout the crystal growth process, which takes place typically over several days at greater than 2000° C., in order to produce a boule having consistent properties throughout. Small variations in process conditions can result in large changes to the quality of the grown silicon carbide boule. Also, as growth proceeds, sublimation of the seed and/or the growing crystal can occur if process conditions are not properly controlled. In addition, product quality can be affected by the types of components used in the crystal growth chamber, since, depending on the growth conditions, some may decompose and thereby chemically interfere with the growth. As a result, silicon carbide grown in a sublimation furnace often contains defects in the crystals, such as low angle grain boundaries, dislocations, Si and C second phase inclusions, different polytype inclusions, and micropipes, which affect the performance properties of the material. Furthermore, even if specific conditions can be maintained for a single crystal growth process to produce a high quality product, run to run variability is also typically seen since, for example, any variability in the source, the seed, or components of the apparatus can produce inconsistencies in the product.
For this reason there has not been to date a reliable and repeatable silicon carbide sublimation furnace or method that can efficiently and cost-effectively produce high quality large silicon carbide single crystals. Therefore, there is a need in the industry for an improved silicon carbide growth apparatus and method.